Wipe the mushrooms (there's no need to peel them).
Add to boiling water for 3-4 minutes, until just tender. Drain well.
Mix the olive oil with the vinegar, garlic, salt and pepper, mustard seeds, chilli flakes, bayleaf and thyme.
Place the mushrooms into a glass jar or non metallic bowl and pour over the marinade. Allow to cool then cover with foil or a tight fitting lid and put in the fridge.
Keep in fridge up to 4 days.
Bring to room temperature before serving.Sprinkle over fresh parsley just before serving, if you wish.